Those skilled in the arts of antennas, antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular circular hollow conductive waveguides, dielectric paths, and strip conductors over ground. and the like. Antennas in principle are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates for transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. In general, small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the phase and apportionment of power to (or from) the antenna elements are termed “beamformers.” In general, a beamformer includes at least one beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals are received by the antenna elements and coupled in guided form to the element ports of the beamformer, and are combined in the beamformer to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different antenna beams may be generated simultaneously.
Array antennas are well known for various communication and sensing purposes, and exhibit advantages over shaped-reflector antennas in that scanning of the beam or beams through spatial angles can be performed essentially instantaneously, without inertia problems associated with the moving of a discrete object. In order to perform its role of setting the direction of the antenna beam of an array, a beamformer must set the element-to-element phase of the signal being transduced. So long as the beam direction is fixed, fixed phase shifting elements may be used in the beamformer to set the element-to-element phase. Such phase shifting elements are ordinarily passive rather than active. Those skilled in the art know that passive elements are ordinarily very reliable. When the shape or direction of the array antenna beam must be controllable rather than fixed, it is customary to use controllable phase shifters in the beamformers. Controllable phase shifters may be analog or digital. Current designs prefer multibit digital types of phase shifters because they can be controlled by simple digital signals, and because the phase shifts can be readily and accurately set.
It will be appreciated that an array antenna, when used with a beamformer for transmitting signal, may require the application of significant power to the beam port(s) of the beamformer in order to achieve the desired power density in the radiated beam so formed. This power tends to be attenuated by the unavoidable “heating” or “ohmic” losses in the beamformer. It may be found to be desirable to distribute relatively small-amplitude signals through the beamformer, and to amplify the signal at the individual element ports of the beamformer before application to each element of the antenna array for transmission. Thus, the array antenna when used for transmission may require a “power” or “transmit” amplifier for each antenna array element or group of elements. When an array antenna is operated in a receiving mode, the signal received by each antenna element must pass through the beamformer before being combined with signals received by other antenna elements. Since the beamformer is subject to losses, the received signal tends to be attenuated by passage through the beamformer. This attenuation tends to undesirably reduce the signal-to-noise ratio of the combined received signal at the beam port of the beamformer. The signal-to-noise ratio can often be improved by amplifying the signal received by each antenna element in a low-noise amplifier before application of the signal to the beamformer.
Selection or control of the antenna beam of an array antenna also involves the “weighting” of the relative power applied to the various antenna elements of the array. The purpose of weighting is to establish the sidelobe level and the distribution of directivity as between the main antenna beam lobe(s) and unwanted subsidiary lobes. An unweighted distribution provides each antenna element of the array with equal weight, meaning that in a transmission mode of operation all antenna elements are excited with equal amplitude signals, and in a receive mode of operation the combining of the signals from the various antenna elements is made with equal amplitude. Such a uniform weighting may be desirable when maximum directivity or gain is desired. However, a uniform aperture distribution tends to result in significant sidelobe levels, which are about −13 dB to −18 dB relative to the peak of the main lobe. The sidelobes may be viewed as being an inherent defect of an antenna, in that they result in transmission of power in directions other than the desired direction, or reception of signals from directions other than that desired. Such transmission in unwanted directions can result in detection of the source of the signals by hostile forces, and in any case represents a waste of transmitter power toward regions of no interest. The reception of signals from undesired directions can expose the antenna to jamming signals from unknown directions. The prior art controls the sidelobe level of an antenna by weighting or adjusting the aperture field distribution. Examples of prior art weighting functions that produce low side lobe levels in the absence of element failures include raised-cosine weighting, Dolph-Tschebyscheff weighting, and Taylor weighting functions. Weighting in an array antenna is ordinarily a function of the beamformer. The distribution of signals in the beamformer may be accomplished by power dividers (or power combiners in receive mode) selected to give the desired array element weighting. In some instances, variable gain or attenuation may be used.
Thus, each antenna element of an array antenna may be associated with a “power” amplifier for use in a transmit mode, a “low-noise” amplifier for use in a receive mode, in addition to at least one phase shifter, and more than one phase shifter if the transmit beam direction may differ from the receive beam direction. The power amplifier, the low-noise amplifier, and the phase shifter(s) associated with each antenna element (or possibly subarray of antenna elements) are often combined into a “transmit-receive” (TR) module. This module, in addition to the amplifiers and phase shifter(s) may also include any controllable gain elements, radio-frequency (RF) switches for switching between transmit and receive modes of operation, controls for the switches, and power supplies for the various controls and active devices.
In the past, the term “radio frequency” was interpreted to mean a limited range of frequencies, such as, for example, the range extending from about 20 KHz to 2 MHz. Those skilled in the art know that “radio” frequencies as now understood extends over the entire frequency spectrum, including those frequencies in the “microwave” and “millimeter-wave” regions, and up to light-wave frequencies. Many of these frequencies are very important for commercial purposes, as they include the frequencies at which radar systems, global positioning systems, satellite cellular communications and ordinary terrestrial cellphone systems operate.
It will be appreciated that an array antenna, especially one containing thousands or tens of thousands of antenna elements, may be physically large. The large physical size, in turn, means that wind loading may impart strong forces to the structure and to the antenna elements themselves. It is common to protect the antenna elements by the use of a “radome.” The “dome” aspect of the term “radome” comes from a time at which physically movable antennas were used for scanning, and the protective radomes were generally at least partially spherical. The radome is intended to protect the antenna elements from the environment, and may be planar if appropriate. Antenna elements which project significantly from their feed points, such as the axial helix antenna elements described in U.S. Pat. No. 5,258,771 issued Nov. 2, 1993 in the name of Praba, may be difficult to protect with a simple radome structure. Praba describes an antenna array operating in two disparate frequency bands, which uses interleaved axial-mode “helical” antenna elements. Each such helical antenna includes an electrically conductive element helically disposed about a longitudinal axis, with a feed point adjacent a ground plane disposed orthogonal to the axis of the helix. Such helical antenna elements are well known, and have the advantage, when so fed against a ground plane, of providing moderately high gain, together with circular polarization. In order to reduce the interaction between the helical antenna elements of the arrays at the disparate frequencies as described by Praba, the helices of the two interleaved arrays are oppositely wound, so that a right-circularly-polarized antenna element is adjacent a left-circularly-polarized antenna element, which results in some degree of rejection of the cross-polarization signal from the adjacent elements, and thereby tends to reduce mutual coupling between the antenna elements of the two interleaved arrays.
Other types of antenna elements can be used in array antennas. So-called “patch” antennas are generally planar, and in the context of an array antenna lie in the plane of the array, without significant projections above (in the radiating direction relative to) the plane of the array. Such patch antennas have the salient advantage of low wind loading, and are easy to protect with a planar radome.
In order to transmit or receive electromagnetic signal, an antenna element must respond to an electromagnetic field traveling toward or from the desired direction. In order to respond to the electromagnetic signal, the antenna must have a finite physical extent or “aperture” in the desired polarization in order to interact with the field being transduced. One of the salient advantages of an arrangement such as that of the Praba patent is that it has finite extent in two dimensions. A planar array of planar patch antenna elements, when viewed from a direction orthogonal to the plane of the array, has a physical extent which substantially equals the patch dimension for the polarization in question. Viewed from a location within the plane of the array, however, each patch antenna has substantially zero projected extent or dimension, at least in one polarization. Consequently, the ability of a planar array of planar or patch antennas to transceive in the direction of the plane may be limited, or in antenna terms it may have relatively low “gain”. In addition to the problem of lack of projected dimension which results in low gain in the plane of the array, there is the problem that radiation to or from any one element of the array must pass by one or more adjacent antenna elements. These adjacent antenna elements tend to interact with so much field as may exist, which in turn tends to “block” the field to or from adjacent antenna elements. This interaction between mutually adjacent antenna elements of an array is termed “mutual coupling.” One manifestation of mutual coupling is a tendency of the impedance of the antenna element to be dependent on the signal transduced by the adjacent (and sometimes semi-adjacent) elements. Mutual coupling often has adverse consequences in the overall operation of the array, and may be undesired.
It is desirable to be able to produce as much radiated power as possible in the transmitting mode of the antenna array in order to maximize the power aperture efficiency. The passage of electromagnetic signal through the radome results in heating of the radome. Since the radome may also be subject to substantial solar loading, it is desirable to reduce the radome temperature.
Improved or alternative antenna arrays and elements therefor are desired.